Amazing video shows what sound looks like

YouTube/NPR, Skunkbear
What if we could see sound waves? The world would look like a fuzzy, vibrating mess.

Sound is normally invisible, but there's a photographic trick that actually lets us see what it looks like. And not just a digital representation of sound waves, but the actual waves themselves — like the sound waves from a person clapping, or a firecracker exploding.

It's possible to see sound waves because they distort the air density around them. NPR's science Tumblr "Skunkbear" put together a video last year showing how the photographic trick works.

The trick is called Schlieran Flow Visualization, and it is actually a pretty simple set up. You start by shining a light through a single slit at a curved mirror.

The light reflects off the first mirror in neat parallel rays. Those straight beams of light bounce off a second curved mirror and converge into a single focal point:

YouTube/NPR, Skunkbear

If you focus that light on a camera lens, you can see a recreated picture of the light source.

But if you interrupt those beams of light with vibrations from sound or with heat, both of which change the density of the air they pass through, something cool happens.

It's easier to think about this in terms of heat first, since you've probably already noticed an example of this on a really hot day when the air above the pavement in the distance looks like it's shimmering. That happens because the heated air has a different density than the cooler air around it, and this higher density interacts with the light differently, bends some of it.

If you put a candle (or anything that generates heat or sound) in between the two mirrors of the original setup, the heat will bend some of those straight beams of light. Then if you place a barrier just above the focal point of light, it blocks the bent light rays from passing through the lens of the camera, and you can see where the air density is different because these areas are darker from less light getting through to the lens:

YouTube/NPR, Skunkbear

The technique can be used to see anything that distorts the air, like the heat from a straightening iron or heat radiating from human skin:

YouTube/NPR, Skunkbear

Like heat, sound waves are just another thing that distorts air density, so the technique can be used to visualize sound waves as they travel through the air. This is what it looks like when you drop a book: